U.S. patent number 6,006,055 [Application Number 09/144,200] was granted by the patent office on 1999-12-21 for image forming apparatus.
This patent grant is currently assigned to Canon Kabushiki Kashia. Invention is credited to Tadashi Furuya.
United States Patent |
6,006,055 |
Furuya |
December 21, 1999 |
**Please see images for:
( Certificate of Correction ) ** |
Image forming apparatus
Abstract
An image forming apparatus includes a movable image bearing
member, a charging member, a developer, and an auxiliary member.
The charging member contacts the image bearing member to
electrically charge the image bearing member. The developer
develops, with toner, an electrostatic image formed on the image
bearing member using a charging operation of the charging member.
The developer is capable or removing residual toner from the image
bearing member simultaneously with its developing operation for the
electrostatic image. The auxiliary member is disposed upstream of
the charging position and downstream of a transfer position where
the toner image is transferred from the image bearing member onto a
transfer material with respect to a movement direction of the image
bearing member. The auxiliary member is supplied with a voltage
having a polarity opposite from a charge polarity of the charging
member or is supplied with OV so that residual toner is charged to
the opposite polarity and reaches the charging position. The
voltage applied to the auxiliary member is more toward the opposit
polarity side when a region of the image bearing member, which is
going to be a non-image region, is at a position of the auxiliary
member then when a region of the image bearing member, which is
going to be an image region, is at the position of the auxiliary
member.
Inventors: |
Furuya; Tadashi (Susono,
JP) |
Assignee: |
Canon Kabushiki Kashia (Tokyo,
JP)
|
Family
ID: |
17197640 |
Appl.
No.: |
09/144,200 |
Filed: |
August 31, 1998 |
Foreign Application Priority Data
|
|
|
|
|
Aug 29, 1997 [JP] |
|
|
9-249748 |
|
Current U.S.
Class: |
399/149;
399/148 |
Current CPC
Class: |
G03G
21/0064 (20130101); G03G 2221/0005 (20130101); G03G
2215/021 (20130101) |
Current International
Class: |
G03G
21/00 (20060101); G03G 015/30 () |
Field of
Search: |
;399/148-150
;361/225,230,214 |
Foreign Patent Documents
Primary Examiner: Smith; Matthew S.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. An image forming apparatus comprising:
a movable image bearing member;
a charging member contactable to said image bearing member to
electrically charge said image bearing member;
developing means for developing with toner an electrostatic image
formed on said image bearing member using charging operation of
said charging member, said developing means being capable of
removing residual toner from said image bearing member
simultaneously with its development operation for the electrostatic
image; and
an auxiliary member, disposed upstream of said charging position
and downstream of a transfer position where the toner image is
transferred from said image bearing member onto a transfer material
with respect to a movement direction of said image bearing member,
said auxiliary member being supplied with a voltage having a
polarity opposite from a charge polarity of said charging member or
supplied with OV so that residual toner is charged to the opposite
polarity and reaches said charging position;
wherein the voltage applied to said auxiliary member is more toward
the opposite polarity side when a region of said image bearing
member which is going to be a non-image region is at a position of
said auxiliary member than when a region of said image bearing
member which is going to be an image region is at the position of
said auxiliary member.
2. An apparatus according to claim 1, wherein the voltage applied
to said auxiliary member when the region of said image bearing
member which is going to be the non-image region is at the position
of said auxiliary member, is selectable from at least two levels of
voltages.
3. An apparatus according to claim 1, wherein said charging member
includes a magnetic brush of magnetic particles contactable to said
image bearing member.
4. An apparatus according to claim 1, wherein said charging member
includes an elastic roller.
5. An apparatus according to claim 1, wherein the charge polarity
of said charging member and a regular charge polarity of the toner
are the same.
6. An apparatus according to claim 1, wherein said auxiliary member
charges toner, having the same polarity as the charge polarity of
said charging member, of the residual toner to the opposite
polarity.
7. An apparatus according to claim 1, wherein said auxiliary member
is contactable to said image bearing member.
Description
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus
including a charging member contactable to an image bearing member
to charge the image bearing member such as a photosensitive member
or dielectric member.
An image forming apparatus such as a transfer type, electrostatic
recording apparatus or the like is known wherein a toner image is
formed through an image formation process including a process of
uniformly charging the image bearing member such as an
electrophotographic photosensitive member, electrostatic recording
dielectric member or the like, and the toner image is transferred
onto a recording material, and the image bearing member is
repeatedly used for the image formation. In such an apparatus, a
cleanerless type has been proposed wherein the cleaning operation
for removing from the image bearing member the residual
untransferred toner after the transfer of the image onto the image
bearing member is effected by the charging device and/or the
developing device(simultaneous charging and cleaning and/or
simultaneous development and cleaning), by which the exclusive
cleaning device is omitted for the purpose of downsizing of the
apparatus.
b) On the other hand, corona charging means has been widely used as
the uniform charging means for the image bearing member. The corona
discharge device is disposed opposed to the image bearing member
without contact thereto, and the image bearing member surface is
exposed to a corona shower discharged from the corona discharge
device to uniformly charge it to a predetermined polarity and
potential.
Recently, from the standpoint of environmental health, a contact
charging device having an advantage of small ozone product, low
voltage and low electric power becomes widely used. In this type,
an electroconductive charging device(contact charging member) in
the form of a roller, blade, fur brush, magnetic brush or the like,
is contacted to the image bearing member, and the contact charging
device is supplied with a predetermined charging bias voltage, so
that image bearing member surface is uniformly charged to a
predetermined polarity and potential.
The charging roller(charging roller, charging roller) is an
electroconductive elastic roller member having a core metal
functioning as an electric energy supply electrode and a rubber
member thereon having an adjusted resistance, and it is contacted
to the surface of the image bearing member as the member to be
charged. It is rotated by a driving system or by the member to be
charged, while a charging bias voltage is applied to the core
metal, by which the image bearing member surface is charged
uniformly to a predetermined polarity and potential.
The charging blade comprises an electrode plate and an
electroconductive elastic material of electroconductive rubber or
the like thereon, and the free end of the blade is contacted to the
surface of the image bearing member as the member to be charged,
and the electrode plate is supplied with a charging bias voltage,
so that image bearing member surface is uniformly charged to a
predetermined polarity and potential.
The fur-brush charging device is a rotary type or non-rotatable
type brush member of electroconductive fibers, and the
electroconductive fiber brush portion(furbrush portion) is
contacted to the surface of the image bearing member as the member
to be charged, and a charging bias voltage is applied, so that
image bearing member surface is uniformly charged to a
predetermined polarity and potential.
The magnetic brush charging apparatus comprises a rotatable or
non-rotatable supporting member functioning also as an electric
energy supply electrode, and magnetic particles (electroconductive
magnetic particles) magnetically confined in the form of a magnetic
brush portion, and the magnetic brush portion of the magnetic
particles is contacted to the surface of the image bearing member
as the member to be charged, and a charging bias voltage is applied
to the supporting member, by which the image bearing member surface
is uniformly charged to a predetermined polarity and potential.
c) A cleanerless type image forming apparatus has been proposed
wherein from the standpoint of both of the downsizing of the image
forming apparatus and the environmental health, the charging of the
image bearing member(photosensitive member) is effected by the
contact charging device, and a cleaning device exclusively for
removal of the untransferred toner from the photosensitive member
surface after toner image transfer onto the recording material, is
not provided, and the untransferred toner is at least temporarily
collected by the contact charging device.
In such an image forming apparatus, the untransferred toner on the
photosensitive member surface after toner image transfer onto the
recording material in the transfer portion, is carried over, by the
continuing movement of the photosensitive member surface, to a
charging portion where the contact charging device is contacted to
the photosensitive member(charging region or charging nip), and is
at least temporarily collected from the photosensitive member
surface to the contact charging device (simultaneous charging and
cleaning). The toner not collected by the contact charging device
and passed through the charging portion and the toner partly
discharged from the contact charging device to the photosensitive
member, are carried to the developing device by the continuing
movement of the photosensitive member surface, and are collected by
the developing device (simultaneous development and cleaning). More
particularly, the toner is collected by a potential difference back
for fog removal which is a potential difference between the DC
voltage applied to the developing device in the next development
operation and the surface potential of the photosensitive
member.
The untransferred toner whose charge polarity is reversed in the
transfer portion by the temporary collection of the untransferred
toner to the contact charging device, are recharged to the regular
charge polarity, and the untransferred toner pattern is scraped, so
that production of the ghost image of the untransferred toner
pattern is prevented.
Such a contact charging type, transfer type and cleanerless type
image forming apparatus is not provided with an independent and
exclusive cleaning device for removal of the untransferred toner
from the photosensitive member surface, and therefore, the
advantage from the standpoint of space is significant, and the
significant downsizing of the apparatus is accomplished, and in
addition, the amount of the ozone product by the discharge is
small. The untransferred toner is collected eventually by the
developing device and is reused in the subsequent process or
processes, so that residual toner can be minimized, and therefore,
it is desirable from the standpoint of environmental health.
It has been proposed in U.S. patent application No. 08/832861 that
in an image forming apparatus of a cleanerless type wherein the
untransferred toner is temporarily collected to the contact
charging device, an auxiliary member is contacted to the
photosensitive member surface, and the auxiliary member being a
fixed furbrush between the transfer portion and the charging
portion at a downstream of the transfer portion with respect to the
movement direction of the photosensitive member surface to enhance
the collection property of the untransferred toner.
The auxiliary member has a function of disturbing the untransferred
toner into a non-pattern form. More positively, it may be supplied
with an opposite bias having a polarity opposite from the charging
bias, so as to trap only the untransferred toner having the same
polarity as the charging bias which is not easily collected by the
charging portion, and change the polarity thereof, and then
discharges it, by which the collection property of the
untransferred toner is improved. By this process, the ghost image
is further reduced.
The deposition and mixing of the toner having a high resistance
into the contact charging device(toner contamination of the contact
charging device) increases the resistance value of the contact
charging device to decrease the charging property.
Therefore, in the image forming apparatus wherein the untransferred
toner is at least temporarily collected by the contact charging
device, it is desirable that toner contamination of the contact
charging device by the collected toner is maintained below an
allowable level.
For example, the magnetic brush charging apparatus is not easily
influenced by the toner contamination as compared with the contact
charging device such as a charging roller or furbrush charging
device or the like. Actually, however, by the imbalance between the
toner amount collected (deposition or mixing) by the magnetic brush
portion and the toner amount discharged from the magnetic brush
portion, the toner amount is integrated in the magnetic brush
portion with repeated operations, even beyond the allowable level
with the result of the decrease of the charging property by the
increase of the resistance value of the magnetic brush portion.
It is known to provide a charging member cleaning mode wherein the
introduced toner is discharged onto the photosensitive member from
the contact charging device during non-image formation in which the
image forming apparatus does not form the image, by which the
toner-contaminated state is maintained at a level lower than the
allowable level(EP-A-766146).
More particularly, the applied voltage to the contact charging
device is adjusted to provide a potential difference .DELTA.V
between the contact charging device and the photosensitive member,
and by the potential difference, the high resistance toner mixed to
the contact charging device is discharged to the photosensitive
member, and is collected by the developing zone.
If, however, the potential difference .DELTA.V is not sufficient,
the introduced toner is not sufficiently discharged with the result
of deterioration of the charging property and therefore the
deteriorated image quality.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to
provide an image forming apparatus wherein the toner discharging
from the charging member to the image bearing member is always
sufficient and assured.
It is another object of the present invention to provide an image
forming apparatus wherein the improper charging due to a large
amount of toner deposited to the charging member, is prevented to
accomplish satisfactory image formation.
It is a further object of the present invention to provide an image
forming apparatus wherein a ghost image is prevented by means of an
auxiliary member.
It is a further object of the present invention to provide an image
forming apparatus wherein movement of the toner from the charging
member to the image bearing member is made easier by means of an
auxiliary member.
These and other objects, features and advantages of the present
invention will become more apparent upon a consideration of the
following description of the preferred embodiments of the present
invention taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of an image forming apparatus
according to a first embodiment of the present invention.
FIG. 2 is a schematic view of a magnetic brush charging apparatus
of a rotatable sleeve type used as a contact charging device in the
image forming apparatus of the first embodiment.
FIG. 3 is a timing chart of an applied bias to an auxiliary member
and a charging member cleaning mode.
FIG. 4 is a schematic illustration of a major part of an image
forming apparatus according to a second embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
(1) Example of image forming apparatus
FIG. 1 is a schematic illustration of an image forming apparatus of
this embodiment. The image forming apparatus of this example is a
magnetic brush charging and cleanerless type laser beam printer
using an image transfer type electrophotographic process.
Designated by 1 is a rotatable drum type electrophotographic
photosensitive member as an image bearing member(member to be
charged). The photosensitive member 1 is rotated at a predetermined
process speed (peripheral speed) in the clockwise direction
indicated by an arrow. In this embodiment, the peripheral speed is
100 mm/sec.
Designated by 2 is a contact charging device contacted to the
photosensitive member 1 (contact charging member). The contact
charging device 2 of the example is a magnetic brush charging
apparatus (magnetic brush charging member) of a rotatable sleeve
type. Designated by n is a charging portion (charging region or
charging nip) where the magnetic brush charging apparatus 2 is
contacted to the photosensitive member 1. The magnetic brush
charging apparatus 2 will be described in section (2)
hereinafter.
The magnetic brush charging apparatus 2 is supplied with a
predetermined charging bias in the form of a sum of a DC bias Vdc
and an AC bias Vac (oscillating voltage which is a voltage having a
voltage level which periodically changes with time) from a charging
bias applying voltage source S1, so that outer surface of the
rotatable photosensitive member 1 is charged uniformly to a
predetermined polarity and potential. The DC bias component Vdc is
substantially equal to the desired charged potential of the member
to be charged, and in this embodiment, the DC bias component Vdc is
-700 V.
The charged surface or the rotatable photosensitive member 1 is
exposed to a laser beam scanning exposure X by a laser scanner 7
(exposure device), so that electrostatic latent image is formed
correspondingly to the intended image information on the peripheral
surface of the rotatable photosensitive member 1.
The laser scanner 7 emits a laser beam having an intensity
modulated by the time series electrical digital pixel signal
corresponding to the intended image information. Designated by 7a
is a mirror for deflecting the output laser beam from the laser
scanner 7 toward the image exposure portion of the photosensitive
member 1.
The electrostatic latent image on the surface of the rotatable
photosensitive member 1 is developed through a reverse development
into a toner image in a developing zone D by a developing device 3,
using insulative toner (negative charged toner) having a negative
charge polarity. Designated by 3a is a developer carrying member in
the form of a developing roller (developing sleeve), and S2 is a
developing bias applying voltage source for applying a
predetermined developing bias to the developing roller 3a. The
developing bias applying voltage source S2 applies a developing
bias in the form of a sum of a DC bias and an AC bias voltages. In
this embodiment, DC bias is -480 V.
On the other hand, a transfer material P as a recording material is
fed from an unshown sheet feeder, and is introduced at a
predetermined timing into a nip (transfer portion)T where a
transfer roller 4 having an intermediate resistance (contact type
transferring device) is contacted to the photosensitive member 1
with a predetermined pressure and with the transfer material
intervening between them. The transfer roller 4 is supplied with a
transfer bias voltage having an opposite polarity from the toner by
the transfer bias application voltage source S3.
The transfer material P introduced to the transfer portion T is
passed through the transfer portion T, during which the toner image
is sequentially transferred from the surface of the photosensitive
member 1 onto the front side of the transfer material P by
electrostatic force and the pressure.
The transfer material P now having the toner image, is separated
from the surface of the photosensitive member 1, and is fed to a
heat fixing type fixing device 5, where the toner image is fixed on
the transfer material P. Finally, it is discharged as a print.
Designated by 6 is an auxiliary member contacted to the surface of
the photosensitive member 1 between the transfer portion T and the
charging portion n downstream of the transfer portion T with
respect to movement direction of the photosensitive member surface.
In this example, it is a stationary fur brush having an
intermediate resistance. The auxiliary member 6 will be described
in detail in section (3) hereinafter.
(2) Magnetic Brush Charging Apparatus 2
FIG. 2 is a schematic view of a magnetic brush charging apparatus 2
of a rotatable sleeve type as the contact charging device, and it
comprises a stationary (non-rotatable) magnet roller 22 as magnetic
field generating means, a rotatable non-magnetic electrode
sleeve(electroconductive base) 21 of aluminum fitted around the
magnet roller 22, and a magnetic brush layer 24 of
electroconductive magnetic particles 23 magnetically attracted on
the outer surface of the electrode sleeve 21 by the magnetic force
of the magnet roller 22 in the sleeve.
The electroconductive magnetic particles 23 constituting the
magnetic brush portion 24 may be ferrite, magnetite or another
magnetic metal particles or such magnetic particles bound by resin
material.
The volume resistivity of the magnetic particle 23 is preferably
1.times.10.sup.6 -10.sup.9 .OMEGA.cm. The particle size thereof is
preferably 10-50 .mu.m. By mixing a plurality of magnetic
particles, the charging property can be improved.
As for a resistance measurement of the magnetic particles, 2 g of
the magnetic particles are placed in a cylindrical container having
a bottom surface area of 228 mm.sup.2, and is pressed by 15 kg. A
voltage of 100 V is applied between the top and the bottom thereof,
and the resistance is calculated from the current, and the obtained
value is regularized.
The average particle size of the magnetic particles, is indicated
by a maximum angular distance in a horizontal direction, and not
less than particles 300 are randomly selected through a microscopic
method, and the diameters thereof are measured, and the arithmetic
average are obtained.
The magnetic brush charging apparatus 2 is disposed substantially
in parallel with the photosensitive member 1 with a gap of 0.5 mm
between the surface of the electrode sleeve 21 and the surface of
the photosensitive member 1 by spacer members (unshown) contacted
to the surface of the photosensitive member 1 at the longitudinal
end portions, so that predetermined width of the magnetic brush
layer 24 is contacted to the photosensitive member 1 surface to
constitute a charging portion n.
The electrode sleeve 21 is rotated at the peripheral speed
substantially equal to the peripheral speed of the photosensitive
member 1 in the clockwise direction indicated by the arrow which is
opposite from the rotational direction of the photosensitive member
1 in the charging portion n, and by this, the magnetic brush layer
24 rubs the surface of the photosensitive member 1 at the
peripheral speed substantially equal to the photosensitive member 1
in the same as the electrode sleeve 21.
By the application of the predetermined charging bias Vdc+Vac
(oscillating voltage) from the charging bias voltage source S1 to
the electrode sleeve 21 of the magnetic brush charging apparatus 2,
the surface of the photosensitive member 1 is charged to the
substantially same potential(substantially -700 V in this example)
as the DC bias component Vdc of the applied charging bias to the
magnetic brush charging apparatus 2, in the charging portion n
through the electroconductive magnetic particles 23 of the magnetic
brush layer 24.
(3) Cleanerless System
Since the printer is a cleanerless printer, there is not provided
an independent cleaner(cleaning device) exclusively for removing
the untransferred toner remaining on the surface of the
photosensitive member 1 after the toner image transfer onto the
transfer material P at the transfer portion T.
As described hereinbefore, a stationary intermediate resistance fur
brush as an auxiliary member is disposed contacted to the surface
of the photosensitive member 1 between the transfer portion T and
the charging portion n downstream of the transfer portion T with
respect to the movement direction of the photosensitive member
surface.
The auxiliary member 6 is supplied with an opposite bias(positive)
which is opposite from the applied charging bias(negative) to the
magnetic brush charging apparatus 2, from the voltage source for
the auxiliary member bias S4.
The untransferred toner remaining on the photosensitive member
surface after the toner image transfer onto the transfer material P
at the transfer portion T, is carried over to the portion E where
the furbrush 6 as the auxiliary member is contacted to the
photosensitive member 1 with the rotation of the photosensitive
member 1, and is disturbed by the fur brush 6 into a non-pattern
form.
The auxiliary member is supplied with a positive bias, so that such
toner of the untransferred toner in the contact portion E as has
the polarity which is the same as the bias applied to the magnetic
brush charging apparatus 2, namely, the toner having the regular
charging polarity(negative toner), is trapped thereby, and is
discharged to the photosensitive member 1 as positive toner. The
toner particles charged to the positive polarity passes through the
contact portion E of the fur brush 6, as it is.
By this, the untransferred toner carried to the charging portion n
are uniformly charged to the opposite polarity (positive) from the
charging bias, and the collection property of the untransferred
toner from the photosensitive member 1 by the magnetic brush
charging apparatus 2 is improved (simultaneous charging and
cleaning). More particularly, since the untransferred toner is
collected temporarily by the charging device 2, the pattern of the
previous image does not remain when the next image is formed.
In this embodiment, the bias applied to the furbrush as the
auxiliary member 6 is +500 V to improve the collection property of
the untransferred toner in the magnetic brush charging apparatus 2
and to reduced the ghost image.
Here, the use is made with an opposite bias(positive) having a
polarity opposite from that of applied bias(negative) applied to
the contact charging device 2, for the bias to be applied to the
auxiliary member 6. However when the transfer bias is so low that
surface potential of the photosensitive member does not become
positive after the image transfer from the photosensitive member 1,
the applied bias to the auxiliary member 6 may be 0, and the
similar advantages are provided. More particularly, when the
potential of the photosensitive member is negative, and the
potential of the auxiliary member is 0V, the potential of the
auxiliary member is positive relative to the potential of the
photosensitive member, so that negative charged toner among the
untransferred toner can be charged to the positive polarity.
The positive untransferred toner collected by the magnetic brush
charging apparatus 2 is charged to the negative polarity (regular
charging polarity) by the friction with the electroconductive
magnetic particle constituting the magnetic brush portion, and then
is uniformly discharged to the surface of the photosensitive member
1 (toner discharge) as negative charged toner. Simultaneously
therewith, the photosensitive member 1 is uniformly charged by the
magnetic brush charging apparatus.
The toner discharged to the surface of the photosensitive member 1
from the magnetic brush charging apparatus 2, is carried to the
developing zone D where the developing device 3 is opposed to the
photosensitive member 1, through the image exposure portion by the
rotation of the photosensitive member 1. In the image exposure
portion, the photosensitive member 1 is exposed to beam from the
laser scanner 7 with the toner remaining thereon to form the
electrostatic latent image.
However, the toner discharged to the surface of the photosensitive
member 1 from the magnetic brush charging apparatus 2 is very
uniformly distributed thereon, and the amount thereof is small, so
that image exposure process is not substantially influenced.
Substantially all of the untransferred toner having discharged from
the magnetic brush charging apparatus 2 to the surface of the
photosensitive member 1 and having reached the developing zone D,
has the regular charging polarity(negative charging), and the toner
deposited on the white background portion among the untransferred
toner on the surface of the photosensitive member 1 in the
developing process is collected into the developing device 3 by a
fog removing electric field((simultaneous development and cleaning)
and is reused. On the other hand, the toner deposited on the image
portion is partly used for the toner image formation. Thus,
simultaneously with formation of the electric field effective to
remove the toner to the developing sleeve from the dark portion of
the electrostatic latent image, the electric field is formed to
deposit the toner to the light portion of the electrostatic latent
image from the developing sleeve.
In this embodiment, a developing bias in the form of a sum of a DC
bias and an AC bias is applied from the developing bias applying
voltage source S2 to the developing roller 3a, wherein the DC bias
is -480 V, and by the potential difference (Vback) between the DC
bias and Vdc of the charging bias, the toner discharged from the
magnetic brush charging apparatus 2 is collected and is reused for
the development.
(4) Charging Member Cleaning Mode
As described hereinbefore, the magnetic brush charging apparatus 2
is not easily influenced by the toner contamination as compared
with the contact charging device such as a charging roller or fur
brush charging device or the like. Actually, however, by the
imbalance between the toner amount collected (deposition or mixing)
by the magnetic brush portion and the toner amount discharged from
the magnetic brush portion, the toner amount is integrated in the
magnetic brush portion with repeated operation, even beyond the
allowable level with the result of the decrease of the charging
property by the increase of the resistance value of the magnetic
brush portion.
Therefore, a charging member cleaning mode is used wherein the
introduced toner is discharged onto the photosensitive member from
the magnetic brush charging apparatus 2 during non-image formation
in which the image forming apparatus does not form the image, by
which the toner-contaminated state is maintained at a level lower
than the allowable level.
In the charging member cleaning mode, the untransferred toner
introduced or mixed to the magnetic brush layer 24 of the magnetic
brush charging apparatus 2 is electrically discharged to the
photosensitive member 1, by which the resistance of the magnetic
brush layer 24 is returned to the original state, thus recovering
the charging property of the magnetic brush charging apparatus 2.
The toner discharged to the photosensitive member 1 is collected by
the downstream developing device 3 with respect to the rotational
direction of the photosensitive member 1, and is reused.
Thus, in the charging member cleaning mode, the toner in the
magnetic brush charging apparatus 2 is discharged at the charging
portion n and is collected by the developing zone D, and therefore,
a relatively large amount of the toner on the photosensitive member
1 is passed through the exposure portion. In order to avoid the
adverse influence of the toner in the exposure portion against the
image exposure of the image formation, the charging member cleaning
mode is carried out mainly during the non-image formation period.
In other words, the toner is moved from the charging member to the
region which is going to be a non-image region of the
photosensitive member.
Here, the operation process of the image forming apparatus
(printer) will be described.
(1) pre-multi-rotation step
Starting operation period (warming period) of image forming
apparatus. In response to actuation of the main switch, the main
motor (unshown) of the apparatus is operated to rotate the
photosensitive member 1 and to carry out the preparing operations
for predetermined process means.
After the start-up operation period, the main motor is once
stopped, and therefore, the rotation of the photosensitive member 1
stops, and the apparatus is kept in a stand-by state until a print
start signal is produced.
(2) pre-rotation process
This is such a period that in response to production of print start
signal, the main motor is reactuated to rerotate the photosensitive
member 1, and preparatory operations are carried out for a certain
period.
(3) printing process (image formation process)
Upon completion of the pre-rotation process, the printing process
including the charging of the rotatable photosensitive member 1,
the image exposure, the development, and the toner image transfer
onto the transfer material or the like through a predetermined
sequence(image forming process), is carried out so that transfer
material P having received the toner image is fed to the fixing
device 5 (first print).
In a continuous printing mode, the printing process is repeated
until the printing process is carried out for the preset
number.
(4) sheet interval stroke
In the continuous printing mode, there is a on-sheet-passing period
of the transfer material P in the transfer portion T, which is a
duration after passing of the trailing edge of a transfer material
P through the transfer portion T and before the leading edge of the
next transfer material P reaches the transfer portion T.
(5) Post-rotation Process
The main motor continues to operate for a certain period after the
completion of the printing process for the n-th (last) sheet, by
which the photosensitive member 1 continues to rotate to carry out
predetermined post-process.
After the completion of the post-rotation process, the main motor
is stopped, and the rotation of the photosensitive member 1 stops.
The apparatus is kept again in the stand-by state until the next
print start signal is produced.
When the printing start signal is produced immediately after the
completion of the pre-multi-rotation, the printing process is
continuously carried out through the pre-rotation process. When
only one sheet is to be printed, the apparatus is placed in the
stand-by state through the post-rotation process, after the
completion of the printing process. Among the above steps, the
period of the printing process(3) is the image formation period,
and the periods of the pre-multi-rotation (1), the pre-rotation
process(2), the sheet interval(4) and the post-rotation process
image forming apparatus, are non-image formation periods.
As shown in FIG. 3, in this example, the charging member cleaning
mode is carried out in the period of the pre-rotation process (2),
the sheet interval (4), and the post-rotation process (5) which are
the non-image formation periods. FIG. 3 deals with a case of two
continuous printings.
In the charging member cleaning mode, the AC component (Vac) of the
charging bias applied to the magnetic brush charging apparatus 2 is
stopped during such non-image-formation periods, so that toner is
electrically discharged from the magnetic brush layer 24 by the
potential difference .DELTA.V between the DC component (Vdc) of the
charging bias applied to the electrode sleeve 21 and the surface
potential (Vd) i.e., (Vdc-Vd) of the charged photosensitive member
1. Alternatively, the peak-to-peak voltage of the AC component of
the charging bias may be lowered to discharge the toner.
By the stop of the AC component (Vac) of the charging bias, the
charging property for the photosensitive member 1 is decreased, so
that surface potential (Vd) thereof is lowered, by which the
potential difference .DELTA.V effective to discharge the toner is
increased. The untransferred toner introduced to the magnetic brush
layer 24 is uniformly charged to the negative polarity by the
friction with the electroconductive magnetic particle 23, and
therefore, the toner is moved or discharged to the photosensitive
member 1 by the potential difference .DELTA.V.
The cleaning means for the contact charging device (contact
charging member) may be any if it can discharge the introduced
toner from the contact charging device. For example, the
discharging may be effected by adjusting the DC component (Vdc)of
the charging bias to be applied to the magnetic brush charging
apparatus 2, or the discharging may be effected by adjusting the
distance between the magnetic brush charging apparatus 2 and the
photosensitive member 1.
As described in the foregoing, in the prior art, if the charging
member cleaning mode operation is not sufficient, the introduced
toner is not sufficiently discharged with the result of
deterioration of the charging property and therefore the
deteriorated image quality.
In consideration of this, this example is such that, as shown in
FIG. 3, the voltage applied to the auxiliary member is strengthened
toward the polarity which is opposite from the charge polarity of
the magnetic brush charging apparatus, in the charging member
cleaning mode. In order to erase the previous image pattern, the
auxiliary member is supplied with a voltage (+500 V) of the
opposite polarity(opposite from the charging device) or 0 V for the
area which is going to be an image formation region of the
photosensitive member, and in order to improve the toner discharge
from the charging member to the photosensitive member in the
cleaning mode, the voltage to the auxiliary member is strengthened
in the direction of the opposite polarity, for the area which is
going to be a non-image formation area or region of the
photosensitive member. More particularly, the voltage applied to
the auxiliary member is more toward the opposite polarity side
(opposite from the charge polarity) when the region on the image
bearing member which is going to be the non-image region is at the
position of the auxiliary member than when the region of the image
bearing member which is going to be the image region is at the
position of the auxiliary member.
By such strengthening, the surface potential of the surface of the
photosensitive member 1 lowers, and therefore, the surface
potential of the photosensitive member Vd when it passes through
the magnetic brush charging apparatus 2 also lowers, so that
potential difference .DELTA.V effective to discharge the toner is
increased, thus improving the cleaning of the magnetic brush
charging apparatus 2.
Actually, the bias applied to the auxiliary member 6 during the
cleaning mode was set at +800 V, and it was confirmed that toner
discharging potential difference .DELTA.V increased from 40 V to 70
V, and the image formation was good for a long term.
The image formation process operation of the image forming
apparatus including the charging member cleaning mode is controlled
by an unshown control system through a predetermined sequence.
The charging member cleaning mode may be carried out at least one
of the pre-rotation process period (2), the sheet interval period
(4) and the post-rotation process period (5). It may be carried out
in the pre-multi-rotation period.
As described in the foregoing, in this example of the image
transfer type image forming apparatus of the cleanerless type using
the magnetic brush charging apparatus 2, the bias applied to the
auxiliary member 6 is altered for the cleaning mode for the
magnetic brush charging apparatus 2, so that cleaning property for
the magnetic brush charging apparatus 2 is improved, by which
satisfactory image formation is accomplished.
In an injection charging type wherein the charge is directly
injected into the photosensitive member surface, the applied bias
Vdc and the charged potential of the photosensitive member Vd are
substantially equal, and therefore, the toner discharging potential
difference .DELTA.V is small, and therefore, the cleaning of the
magnetic brush charging apparatus 2 has not been sufficient. With
the present invention, the effects were sufficient in the injection
charging type image forming apparatus. In an injection charging
type, the photosensitive member preferably has an injection layer
having a volume resistivity of 10.sup.9 -10.sup.14 .OMEGA.cm on the
photosensitive layer.
Second Embodiment
FIG. 4 is a schematic illustration of a major part of an image
forming apparatus of this embodiment. The hardware structures of
the image forming apparatus of this embodiment is the same as the
image forming apparatus of the above-described first embodiment
(the cleanerless type laser beam printer using the image transfer
type electrophotographic process and the magnetic brush charging)
except that charging roller 2A is used in place to the magnetic
brush charging apparatus 2 as the contact charging device.
The charging roller 2A used in this example comprises a core metal
(electroconductive base) 25 for supplying a charging bias and an
urethane foam rubber layer 26 bonded thereon and abraded into a
roller shape.
The urethane foam rubber layer 26 has a foam diameter of approx.
100 .mu.m, and a hardness of 40.degree. in Asker C hardness. Carbon
is mixed into the rubber to provide a resistivity of 10.sup.5
-10.sup.9 .OMEGA.cm approx. Here, the resistivity is measured in
this manner. The charging roller 2A is contacted to an aluminum
drum and is rotated, while supplying a DC bias of 100 V, and the
resistivity is calculated from the current at that time.
The charging roller 2A is press-contacted to the photosensitive
member 1 with a predetermined pressure against the elasticity of
the urethane rubber layer 26, and is disposed parallel with the
axis of the photosensitive member 1. Designated by a is a charging
portion (charging region or charging nip) where the charging roller
2A is contacted to the photosensitive member 1. The charging roller
2A is rotated counterdirectionally with the peripheral movement
direction of the photosensitive member 1 at the charging portion n
in this example, and is contact thereto with a peripheral speed
difference between the surface of the photosensitive member 1. The
charging roller 2A may be driven by the photosensitive member 1, or
it may be rotated codirectionally with the rotation of the
photosensitive member.
The core metal 25 of the charging roller 2A is supplied with a
predetermined charging bias from a voltage source S1 so that
surface of the rotatable photosensitive member 1 is
contact-charged.
In an image forming apparatus of the cleanerless type using such a
charging roller 2A, the toner introduced to the charging device is
deposited to the surface of the roller, as contrasted to the
magnetic brush charging apparatus 2 of the image forming apparatus
according to the first embodiment, and therefore, the charging
property immediately lowers.
In this example, in consideration of this, there are provided two
levels of bias voltage to be applied to the auxiliary member 6
during the charging member cleaning mode, and the applied bias
value to the auxiliary member 6 is changed in accordance with the
toner amount deposited to the charging roller 2A.
The toner amount deposited to the charging roller 2A, namely, the
degree of the toner contamination of the charging roller 2A, is
related with the charging current. In this example, therefore, the
charging current is measured by a control circuit 8, and the bias
level to be applied to the auxiliary member 6 from the voltage
source S4 during the charging member cleaning mode, is changed in
accordance with the measured charging current. For the region of
the photosensitive member which is going to be an image formation
region, the auxiliary member is supplied with +500 V, similarly to
first embodiment.
More particularly, in this example, the charging current is
measured by a control circuit 8, and when the charging current is
threshold value of 20 .mu.A or higher, the applied bias to the
auxiliary member 6 from the voltage source S4 during the charging
member cleaning mode is set to +800 V, and when it is lower than 20
.mu.A, the applied bias to the auxiliary member 6 is set at +1200
V, by which the cleaning property of the charging roller 2A is
improved. The voltage applied to the auxiliary member is more
toward the opposite polarity side (opposite from the charge
polarity) when the region on the photosensitive member which is
going to be the non-image region is at the position of the
auxiliary member than when the region of the photosensitive member
which is going to be the image region is at the position of the
auxiliary member.
By this, the cleaning property is so improved that good images are
provide even after 2000 sheets are printed (in the prior art, image
defect occurs after 200 sheets print).
In this example, one threshold level(20 .mu.A), and two levels for
the bias applied to the auxiliary member (+800 V, +1200 V), are
provided, but more levels may be provided for each of them. In this
example, the urethane rubber is used for the material of the
charging roller 2A, but this is not limiting, and if the
above-described resistance range is satisfied, various materials
are usable. For example, EPDM rubber or NBR rubber are usable.
In this example, the toner amount (degree of toner contamination of
the charging device) is detected using the charging current, but
this is not limiting, and any detection method is usable if it can
detect the state of toner contamination of the charging device. For
example, in an image forming apparatus using a digital signal, the
consumption of toner may be calculated using the signal, and the
toner amount introduced into the charging device can be estimated
therefrom.
Others
1) The magnetic brush charging apparatus 2 as the contact charging
device is not limited to the rotatable sleeve type. Other examples
include a rotatable magnet roller, or a sleeveless type wherein the
surface of the magnet roller is treated for electroconductivity and
is used as an electric energy supply electrode, and the
electroconductive magnetic particles are directly confined on the
outer surface of the magnet roller to form a magnetic brush
portion, and wherein the magnet roller is rotated. A nonrotatable
magnetic brush member is also usable.
The contact charging device is not limited to the above-described
magnetic brush charging apparatus 2 or the charging roller 2A, but
a charging blade, fur brush charging device or the like is
usable.
2) The auxiliary member 6 is not limited to a stationary furbrush,
but may be a rotatable roller type. The material thereof is not
limited to the fur brush, but another material such as a stationary
or rotatable electroconductive sponge member is usable. The
auxiliary member may be a corona discharger to change the polarity
of the residual toner to the positive polarity from the negative
polarity.
3) The voltage waveform of the AC bias to the contact charging
device 2 or 2A or the developing device or the like, may be a
sinusoidal wave, a rectangular wave, a triangular wave or the like.
Also, the alternating current may be constituted of an alternating
current in the rectangular form which is generated by periodically
turning on and off a DC power source. In other words, the waveform
of the alternating voltage applied, as the charge bias, to a
charging member or a development member may be optional as long as
the voltage value periodically changes.
4) The image exposure means for the electrostatic latent image
formation is not limited to the digital means, but may be analog
image exposure means or may use LED or another light emitting
element. It may be a combination of a light emission element such
as a fluorescent lamp and a liquid crystal shutter or the like, if
an electrostatic latent image can be formed corresponding to the
image information.
The image bearing member may be an electrostatic recording
dielectric member or the like. In such a case, the dielectric
member surface is uniformly charged (primary charging) to a
predetermined polarity and potential, and thereafter, the selective
discharging is effected by discharging needle head, electron gun or
another discharging means to form an intended electrostatic latent
image.
5) The developing system or the structure of the developing device
may be another one. A regular developing means is usable.
6) The recording material for receiving the developer image from
the image bearing member 1 may be an intermediary transfer member
such as a transfer drum.
While the invention has been described with reference to the
structures disclosed herein, it is not confined to the details set
forth and this application is intended to cover such modifications
or changes as may come within the purposes of the improvements or
the scope of the following claims.
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